1. ISIS Second Target Station
Project Summary
Target design, analysis and optimisation
Robbie Scott
Mechanical Design / Project Engineer
ISIS Facility

2. ISIS Second Target Station
Upgrade of ISIS – accelerator based, pulsed neutron source
Synchrotron accelerator shared between both target stations
Double the number of instruments

3. ISIS Second Target Station
Designed for key future scientific needs:
Soft matter
Advanced materials
Bio-molecular science
Nano-technology
Scientific requirements imply need for specific flux characteristics:
Significantly enhanced cold neutron flux
Broad spectral range
High resolution
Moderators designed to provide excellent conditions for required flux characteristics:
Low frequency :
10Hz
100ms frame
Low power:
48kW
60µA
} Wide dynamic range
} Optimised for cold neutron production

5. Neutronically efficient target design
Maximise use of ‘target’ materials
Design target geometry to match proton beam
Maximises target neutron yield, while minimising absorption
Optimise cross-sectional area
Minimise volume of coolant channels
Maximises solid angle which moderators view

6. Baseline target design
Flow Divider
Tungsten Core
D20 Out
EPB
Proton beam window
D20 in
Tantalum Cladding
Stainless Steel Pressure Vessel
D20

7. Optimisation of baseline target design
Reduction in pressure vessel wall by 70%
Reduction in coolant channel depth by 80%
Overall reduction in Target diameter of 28%
Allows moderators to move closer to neutron producing core
Increases solid angle which moderators view
Reduces probability of neutron absorption within target
Resulted in significant increases in neutron flux (60%)

8. Back to the drawing board!
Removal of proton beam window & introduction of new cooling channel concept
Proton beam no longer passes through Inconel window and D20
Flow channel geometry altered – purely radial cooling
Flux increase of approximately 5%
Improved reliability
Pressure distribution within target cooling channels

10. Materials Pressure vessel material choice
Replace stainless steel with Tantalum
Further reductions in target diameter (now 63% of original size)
Further 15% flux increase

11. Consequences of design alterations
Total predicted flux increase due to design alterations ≈ %
Neutron flux 95% of pure Tungsten target
However new cooling channel concept must be proven
Computational Fluid Dynamics (CFD) employed for analysis and optimisation of coolant channels
CFD subsequently verified using flow tests

12. CFD Analysis of initial design
CFX used to Computational Fluid Dynamics analysis
CFD revealed problematic separation & pressure drop at inside of bend
Resulting recirculation would heat coolant excessively

13. Removal of recirculation
A solution was required to remove the recirculation
The flow guide was modified into an aerofoil form
Prevents separation and subsequent recirculation

14. Cavitation A fluid’s vapour pressure is proportional to temperature
If the pressure within a flow falls below the local vapour pressure, cavities (or bubbles) will form
As the cavities leave the low pressure region, they collapse, damaging the vessel wall
2.4 bar 1
-0.7

15. Vapour Pressure H20
Pressure [Pa]

16. Cavitation Prevention
High flow velocities within the target cause a pressure drop on the inside of the bend
If local vapour pressure is greater than local pressure, cavitation will occur
Solution
Map vapour pressure onto flow model
Increase inlet and outlet pressures (maintaining differential) until pressure in all regions are above local vapour pressure
Final inlet pressure 5 bar

17. Modelling proton beam heat load within the target
[K]
MCNPX used to calculate energy deposition by the proton beam within target
Curve fitting allowed the creation of functions which accurately describe the axial and radial variation of heat load

18. Thermally induced stress
Temperatures within target are calculated using CFD
Temperatures exported to an FEA package (ANSYS)
Thermally induced expansions are then calculated
Resultant stresses and are then calculated
Differing coefficient of thermal expansion
Tungsten & Tantalum differ by 2µm/m/°C
Small stresses

19. Verifying CFD Results
Prototype thermal test target, installed with a dense network of pressure tappings
5 cartridge heaters will supply 37kW of power, to test the cooling
Power varied axially along the target

20. Manufacturing Majority of target simple to manufacture:
Tungsten core is encased in a 1mm sleeve of Tantalum
Sleeve is e-beam welded, creating a hermetic seal
Assembly is hot isostatically pressed (HIP)
Ultrasonic NDT used to test HIP bond

21. Tantalum pressure vessel complex to manufacture
Incorporates aerofoil structures on ID!
Former created on CNC mill
Hot Isostatic Pressing is used to create the vessel from powder
Former leached away after vessel created
Pressure vessel shrink fitted onto core, then assembly e-beam welded

22. Project Uncertainties
Potential for erosion due to high coolant velocities
Pressure vessel manufacturing method yet to be proven